FIGS. 1A and 1B diagrammatically illustrate the key problems in ground-based laser communications with satellites and high altitude platforms. Clouds and moisture in the atmosphere scatter the laser beam energy, resulting in lost communications. Wind-borne turbulence refracts, distorts and corrupts the beam, making it difficult for laser communications. There is technology which exists that permits transmission and reception of laser signals. Approaches to communications between the ground source and space or satellite and high altitude platforms that have been postulated require adaptive optics and special signal compensation to allow for communications.
The object of the present invention is to provide a laser communication system for communication between the earth and earth-orbiting satellites or high altitude platforms that does not depend on adaptive optics or special signal compensation to allow laser communications between the ground and the space receiver.
The invention is predicated on the discovery that scintillation "windows", which occur about fifty percent of the time for up to 10 milliseconds, can be used to provide laser communications with a satellite. The present invention is based on the concept of causing a ground-based laser communications transmitter to continuously repeat large data block signals aimed at an earth-orbiting satellite or high altitude platforms and providing a laser communications receiver and acknowledgement transmitter on the earth-orbiting satellites or platform. During the scintillation windows, packetized portions of the large data block signals are received through the window, and on receipt of the packets at the satellite or high altitude platform, an acknowledgement transmitter mounted on the earth-orbiting satellite or platform transmits an acknowledgement signal causing the ground terminal laser communication transmitter to again continuously repeatedly transmit the next succeeding large block data signals and repeating this process until all the data has been received by the earth-orbiting satellite or high altitude platform. Thus, high data rate communication is achieved between space or high altitude platforms and ground terminals using laser communications through the scintillation windows in the atmosphere.
Thus, the limitations on high data rate communications between high altitude space and ground terminals, namely, (1) the atmospheric limitations on coherent bandwidth and (2) frequency use due to international treaties and U.S. government regulations are avoided. These limitations restrict present user bandwidth to a few gigabits in the radio frequency spectrum.
Research and development activities in the use of laser communications have the potential to support several hundred gigabits of communications but are fundamentally limited by atmospheric turbulence, clouds, rainfall and impose extreme stability requirements on both the satellite and the ground terminals. Normal atmospheric conditions create very high loss signal fade so as to cause large signal losses such that long duration communications is not feasible without significant compensation. However, the present invention breaks the data stream into short packets which will fit entirely in the gaps between deep scintillation fades. These packets will be retransmitted multiple times until there is high probability of passing through the gaps defined by the scintillation windows. As noted above, scintillation produces gains in received signal strength about fifty percent of the time and fades about fifty percent of the time. The received signal strength fluctuates randomly up and down with an average period of several milliseconds. The packet length will be somewhat less on the order of tenths of milliseconds. The very high data rates possible with laser communication will yield useful data rates even after reduction due to the required retransmission. As this invention does not require multiple apertures or compensation to mitigate scintillation, it is an affordable solution that still provides the inherent advantages of laser communications. These advantages include small terminals, low probability of interference, and low probability of interception.